Scale robust feature-based identifiers for image identification

ABSTRACT

A method for deriving an image identifier comprises deriving a scale-space representation of an image, and processing the scale-space representation to detect a plurality of feature points having values that are maxima or minima. A representation is derived for a scale-dependent image region associated with one or more of the detected plurality of feature points. In an embodiment, the size of the image region is dependent on the scale associated with the corresponding feature point. An image identifier is derived using the representations derived for the scale-dependent image regions. The image identifiers may be used in a method for comparing images.

BACKGROUND TO THE INVENTION

This invention relates to the problem of robust image identification anddiscloses a new method that offers advantages over prior art methodssuch as those disclosed in the earlier filed, co-pending patentapplications EP 06255239.3, GB 0700468.2, GB 0712388.8, GB 0719833.6 andGB 0800364.2.

DESCRIPTION OF THE RELATED ART

The aim of robust identification of multimedia content is to extractinformation that is preserved under typical modifications that may beperformed on it. That modification may be for innocent or maliciouspurposes. In the case of images, a robust identifier should besubstantially preserved when subjected to compression, greyscaletransformation, rotation, noise, colour alteration, cropping andtranslation amongst others. Additionally, an identifier should providediscrimination powers such that independent content has sufficientlydifferent identifiers.

The techniques disclosed in European patent application No. EP06255239.3, and UK patent application Nos. GB 0700468.2, GB 0712388.8and GB 0719833.6 are similar in that they all extract an identifierbased on global information, that is information about the whole image.This family of related identifiers provide robustness close to 100% atvery low false alarm rates (which means they provide high discriminationability), with typical rates being less than 0.1 parts per million.

Large numbers of images exist in various image databases, embedded inweb-pages, presentation and documents. A survey of prominent web sitesshows that Flickr has over 2 billion images, Photobucket has over 4Billion and Facebook has 1.7 Billion. Consumers often have thousands ofphotographs in their personal collections and professionals will havesignificantly more. To be of practical use given the large size ofdatasets, identifiers must also allow for very fast searching. Thetechniques disclosed in patent applications EP 06255239.3, GB 0700468.2,GB 0712388.8 and GB 0719833.6 may allow for search speeds of over 10million images per second.

Despite high levels of robustness to many common image modifications theprior art identifiers disclosed in patent applications EP 06255239.3, GB0700468.2, GB 0712388.8 and GB 0719833.6 may not provide the requiredlevel of robustness to a class of image modifications, where asignificant part of image content is lost or replaced, such astranslation and cropping. The technique disclosed in GB 0800364.2provides significantly improved robustness under this class of imagemodifications. It is a process that involves i) feature detection andselection, ii) feature region and identifier extraction and iii)matching. The present invention proposes an improvement on therobustness of the prior art image identifiers.

Prior art feature point methods have been developed in the areas ofcomputer vision and object recognition [references 2, 3 infra]. Theapproaches rely on a very large number of points:

-   -   “An important aspect of this approach is that it generates large        numbers of features that densely cover the image . . . . A        typical image of size 500×500 pixels will give rise to about        2000 features.” [reference 2 infra]

The result of extracting large numbers of features is that searching andmatching calculations become very slow; in the naïve case all featuresfrom a query image should be compared with all features from all imagesin the database. In order to reduce the search times prior art methodsuse various forms of data-structures, however these structures come atthe cost of additional database initialisation times, additional storageand memory requirements and often some approximations are made whichtrades improved search speed for lower detection rates.

The present invention is concerned with detecting images and theirmodified versions, which differs from the area of application of much ofthe prior art. In particular, image duplicate detection is concernedwith finding modified versions of an image e.g. cropped, compressed etc.In contrast, most prior art methods are concerned with detectingspecific objects or a class of objects in images, e.g. a Ford Focus caror cars. Whilst the distinction between these problems may appeartrivial or subtle it is important. Consider, for example, two images ofSt Paul's Cathedral in London, the images are taken from two differentview points. Now under the object recognition task the similaritybetween these images should be detected but under the image duplicatedetection the similarity should not be detected. With this in minddifferent techniques are needed to deal with the two different problems.

The method disclosed in GB 0800364.2 develops a binary feature pointdescriptor based on the Trace transform. The advantage of using a binarydescriptor is that the Hamming distance is used for feature matching,which makes the searching process very fast. Furthermore, a selectionprocess limits the number of features to a small set that proves to besurprisingly robust to difficult image modifications such as croppingand translation. Whilst the robustness is good it is desirable toprovide further improvements.

SUMMARY OF THE INVENTION

In accordance with to a first aspect, the present invention provides amethod for deriving a representation of an image, i.e. an imagedescriptor, by processing signals corresponding to the image. The methodcomprises deriving a scale-space representation of the image. Thescale-space representation is processed to detect a plurality of featurepoints in the scale-space representation having amplitude extrema (i.e.maxima or minima values). For one or more of the detected plurality offeature points, the method determines a scale-dependent image regionassociated with the corresponding feature point. A representation foreach determined, scale-dependent image region is then derived.

Thus, an image descriptor may be derived, based on the features of theimage having the highest feature strength.

In an embodiment, the plurality of feature points are detected byapplying a first operator based on second order image gradients, to thescale-space representation of the image, and identifying those featurepoints in the scale-space representation with the highest resultingvalues. A second operator may then be applied to the detected pluralityof feature points, the second operator based on first order imagegradients, and features points with resulting values below a thresholdremoved. A predetermined number of features points may be selected fromthe plurality of features points detected by applying the first andsecond operators.

In other embodiments, the plurality of feature points may be detectedusing the operators in a different order, or using different techniques,according to the desired application.

Preferably, for selected one or more detected features points, ascale-dependent image region is determined by determining a spatialregion of the image associated with the spatial position of the featurepoint, wherein the size of the spatial region is based on the scale ofthe feature point.

A representation for each determined, scale-dependent image region, maybe derived based on line projections across the image region, such as byusing the Trace transform, or equivalent.

In one example, for each of the one or more determined scale-dependentimage regions, a function of at least part of the image region isderived, where the function of a translated, scaled or rotated versionof an image region is a translated or scaled version of the function ofthe image region. The step of deriving a function of at least part ofthe image region may comprise applying a first functional along linesover the image region. The method may further comprise applying a secondfunctional to the derived function to obtain an intermediaterepresentation of the at least part of the image region.

A representation for each determined, scale-dependent image region maycomprise a binary image descriptor.

A representation of the image may be derived from two or more of thederived representations of the scale-dependent image regions.

In accordance with a second aspect, the present invention provides amethod for comparing images using descriptors of images derived using amethod according to the first aspect of the present invention. Themethod comprises comparing representations of one or morescale-dependent image regions associated with the descriptor of a firstimage, with representations of one or more scale-dependent image regionsassociated with the descriptor of a second image.

Further aspects of the present invention include: use, includingtransmission or reception, of a representation of an image derived usinga method according to the first aspect of the present invention; anapparatus for executing a method according to the first aspect or thesecond aspect of the present invention, or both, and a computer-readablestorage medium comprising instructions that, when executed by acomputed, perform a method according to the first aspect or the secondaspect of the present invention, or both.

Other features and advantages of the invention will be apparent from thefollowing description and accompanying claims.

This invention includes two main aspects that improve the robustness asrequired, i) region-scale is incorporated into the process of featureselection and extraction and ii) a matching process of up to four stagesis developed that maintains robustness whilst minimising computationalcost.

A feature detection process is used to select a plurality of features,having a spatial and scale location. The feature selection uses a novelprocess to choose features appropriate to the task and that have beenshown experimentally to work well with Trace transform based featuredescriptors.

For image matching a trade off is needed between detection performance,independence, computational time and memory requirements. In thedisclosed embodiment, a four step matching process is introduced whichaims to maximise detection rates and independence, whilst minimisingboth computational time and memory requirements. The first stage quicklydetermines hypotheses (i.e. candidates) with high probability andeliminates low probability matches, the second stage tests thesehypotheses and removes a significant number of false hypothesis. Thethird stage eliminates further false hypotheses. Each stage involves anincreased level of complexity over the previous stages, so complexity ismanaged and minimised by eliminating false hypotheses as early aspossible in the process.

As well as the aforementioned aspects, other aspects of the inventioninclude deriving a representation of an image based on the selected andextracted features of the image, and an apparatus and a computer programfor implementing processes according to one or more aspects of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, of which:

FIG. 1 a shows an image;

FIG. 1 b shows a reduced version of the image of FIG. 1 a;

FIG. 1 c shows a rotated version of the image of FIG. 1 a;

FIG. 1 d shows a blurred version of the image of FIG. 1 a;

FIG. 1 e shows a flipped (left-right) version of the image of FIG. 1 a;

FIG. 1 f shows a heavily compressed version of the image of FIG. 1 a;

FIG. 1 g shows a cropped version of the image of FIG. 1 a;

FIG. 2 shows an image and a bit string representation of the imageaccording to the prior art;

FIG. 3 shows an example set of interest points of an image detected by afeature detector;

FIG. 4 shows an image from which a final set of features is chosen, thescale of the feature is indicated by the size of the circle and thedirection of the feature is indicated by the line;

FIG. 5 shows interest points found to match using the Hamming distancefrom (a) an original image and (b) a rotated version of (a), in whichthe line lengths between the interest points are used to apply geometricconstraints;

FIG. 6 is a block diagram of an apparatus according to an embodiment ofthe present invention, and

FIG. 7 is a diagram showing the structure of a feature identifier (a)and an image identifier (b) according to and embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Various embodiments for deriving a representation of an image,specifically an image identifier, and for using such arepresentation/identifier for the purposes of, for example,identification, matching or validation of an image or images, will bedescribed below. The present invention is especially useful for, but isnot restricted to, identifying an image. In the described embodiments,an “image identifier” (also referred to as “identifier”, “descriptor”,“image descriptor”, “signature” or “image signature”) is an example of arepresentation of an image and the term is used merely to denote arepresentation of an image, or descriptor.

The skilled person will appreciate that the specific design details ofan image identification apparatus and method, according to an embodimentof the invention, and the derivation of an image identifier for use inimage identification, is determined by the requirements related to thetype of image modifications it should be robust to, the size of theidentifier, extraction and matching complexity, target false-alarm rate,etc.

The following example illustrates a generic design that results in anidentifier that is robust to the following modifications to an image(this is not an exhaustive list):

-   -   Colour reduction,    -   Blurring,    -   Brightness Change,    -   Flip (left-right & top-bottom),    -   Greyscale Conversion,    -   Histogram Equalisation,    -   JPEG Compression,    -   Noise,    -   Rotation,    -   Cropping,    -   Scaling,    -   Translation,    -   Skewing and    -   Perspective change.

It has been found that the proposed design may typically achieve a lowfalse-alarm rate of less than 1 parts per million (ppm) on a broad classof images and typical detection rates above 98.0%.

FIG. 1 shows an example of an image and modified versions of the image.More specifically, FIG. 1 a is an original image, FIG. 1 b is a reduced(scaled) version of the image of FIG. 1 a, FIG. 1 c is a rotated versionof the image of FIG. 1 a, FIG. 1 d is a blurred version of the image ofFIG. 1 a, FIG. 1 e is a flipped version of the image of FIG. 1 a, FIG. 1f is a compressed version of the image of FIG. 1 a and FIG. 1 g is acropped version of the image of FIG. 1 a.

An embodiment of the invention derives a representation of an image, andmore specifically, an image identifier, by processing signals and/orimage data corresponding to the image.

Feature Point Detection and Selection

Consistent with EP 06255239.3, GB 0700468.2, GB 0712388.8, GB 0719833.6and GB 0800364.2, in the initial stage of extraction of the imageidentifier the image is optionally pre-processed by resizing andfiltering. The resizing step is used to normalise the images beforeprocessing. The filtering step can comprise of filtering to removeeffects such as aliasing it can also include region selection andtapering. In one embodiment the image is resized to a resolution of256×N or N×256, where N≧256 and preserving the aspect ratio. In anotherembodiment the image is resized to a square of 256×256. The image isthen low pass filtered with a 3×3 Gaussian kernel. The pre-processingsteps are optional and can include any combination of the above.

In the next stage feature points are detected in the image. Oneembodiment uses a feature detector to detect potential features in animage and their strength based on measuring image gradients. Onesuitable image feature detector based on first order image gradients isthe Harris as used in GB 0800364.2(I _(x) ²(x,y)I _(y) ²(x,y)−I _(x) I _(y)(x,y)I _(x) I _(y)(x,y))−α(I_(x) ²(x,y)+I _(y) ²(x,y))²,  (1)where I_(x) (x, y) is the first order derivative of the image intensityI(x, y) at location (x, y) and α is a parameter that controls featuredetection.

An alternative second order detector is the Laplacian (which correspondsto the trace of the second order partial derivative Hessian matrix)|I _(xx)(x,y)+I _(yy)(x,y)|,  (2)where I_(xx)(x, y) is the second order derivative of the image atlocation (x y).

In GB 0800364.2 the features are selected at just one scale, that is thenormalised version of the original image. Furthermore the area aroundthe feature, which forms an image region from which a feature signatureis extracted, is fixed. However, under the cropping modification, partof the image is lost. When the image is geometrically normalised (as inthe aforementioned pre-processing or by a user) a region becomes largerrelative to the same region in the original image. Under heavy croppingthe Trace transform based descriptors of these two regions becomesunacceptably different; the distance may be above the threshold fordeclaring similarity.

In order to address this problem, in accordance with a first aspect ofthe present invention, “scale estimation” may be used. Scale estimationattempts to select an appropriate scale for analysis of each feature. Itshould be noted that each feature will be associated with its own scale.

First, a scale-space representation of an image is derived. Inembodiments, a scale-space representation of an image may be constructedin one of two ways (although other techniques are possible andcontemplated). The first is to successively smooth and sample theoriginal image, the second is to repeatedly smooth the image withincreasing filter size. Features are then detected by finding maxima inthe location-scale space. It has been shown that an optimal choice forthe smoothing filter is a Gaussian (filter), an image smoothed by aGaussian (filter) is denotedJ(x,y,σ _(D))=G(σ_(D))*I(x,y),  (3)where * is the convolution operator and G(σ_(D)) is a Gaussian filterand σ_(D) is the standard deviation of the smoothing filter thatdetermines the scales.

In one embodiment, two detectors are used to provide good detection ofthe location and scale of features in the scale-space representation,one detector uses first order gradients, the other uses second ordergradients.

Firstly, the modified scale corrected Laplacian of Gaussian operatorσ_(D) ^(γ2) |G(σ_(I))*J _(xx)(x,y,σ _(D))+G(σ_(I))*J _(yy)(x,y,σ_(D))|,  (4)is used to detect a plurality of feature locations and scales (i.e.feature points in the scale-space representation). Where G(σ_(I)) is theGaussian filter used for integration that averages the derivatives,σ_(I) is the integration scale and γ is a scale normalisation parameter.The introduction of integration smoothing to the scale adapted Laplacianhelps to improve the performance of the feature detector.

The second detector is the scale-adapted Harris operator(R _(x) ²(x,y,σ _(D),σ_(I))R _(y) ²(x,y,σ _(D),σ_(I))−R _(x) R_(y)(x,y,σ _(D),σ_(I))R _(x) R _(y)(x,y,σ _(D),σ_(I)))−α(R _(x) ²(x,y,σ_(D),σ_(I))+R _(y) ²(x,y,σ _(D),σ_(I)))²,  (5)whereR _(x)(x,y,σ _(D),σ_(I))=G(σ_(I))*J _(x)(x,y,σ _(D)).  (6)

Accordingly, the Laplacian equation (4) applied in scale space, and thelocations and scales with the highest value are then chosen as potentialfeatures. Then those features with a non-significant (i.e. below athreshold) Harris detector response are removed from the set ofpotential features. A selection procedure as described below is appliedto select a representative set of features from the set of potentialfeatures resulting from the use of the two detectors. In an embodiment,up to a predetermined number of features are chosen.

The strongest feature, according to the Laplacian detector equation (4),from the allowable features amongst the detected features is selectedfirst. A feature is rejected if it is closer to the edge of the imagethan the radius r computed from the feature scale, see equation (7). Thesubsequent features are selected by order of strength with therestriction that a feature is not selected if it is within an exclusionzone around any of the previously selected features. In one embodiment,a circular region with a radius of 10 pixels is used as the exclusionzone. The selection process terminates once a predetermined number ofrepresentative features (e.g. 32 features) has been selected or no morefeatures remain.

A set of features of the image of FIG. 2, selected from those detected,as shown in FIG. 3, is shown in FIG. 4, the scale is represented by thesize of the circle and the feature direction is represented by the line.It will be appreciated that the detected features and their surroundingimage regions are representative of, or significant to, the image as awhole.

Now that the location of the feature (or interest) points has beenselected, their position is preferably refined at the selected scale tosub-pixel resolution using for example a method such as described inOpenCV (FindCornerSubPix) Mar. 1, 2008, which can be obtained on theInternet at http://opencvlibrary.sourceforge.net/CvReference, reference[1] infra).

Feature Region & Identifier Extraction

In GB 0719833.6, incorporated herein by reference, a method forextracting a binary image identifier or image representation isdisclosed. In an embodiment of the present invention, the method of GB0719833.6 is applied to extract a binary “feature identifier” fromregions around feature points. Thus, each feature identifier is a binaryrepresentation of an image region associated with a detected feature.The image region may have a predetermined size. For example, in oneembodiment a circular image region, having a radius of 32 pixels,centred on a feature point, is selected and the binary signature iscomputed for the image region. The circular region is preferablyextracted to sub-pixel level accuracy, using cubic interpolation, forexample.

Preferably, instead of using a fixed feature region radius, the scale atwhich the feature is detected is used to adapt the region sizeappropriately. The radius of the region to be selected at scales isgiven by:r=r ₀σ_(D), σ_(D)=κσ₀ ^(s)  (7)where r₀ is the base radius, σ₀ is the initial scale and κ is a constantthat is a design parameter. It is also worth noting that a good choicefor the integration scale isσ_(I)=σ₀ ^(s).

In a preferred embodiment κ=0.6, σ₀=1.14, r₀=20 s ε(0, 1, . . . , S) andS=14. The region determined by the feature location (x, y) and radius ris extracted to sub-pixel accuracy, using cubic interpolation. Theregion is then normalised to a predefined size, given by the normalisedradius {circumflex over (r)} which is 32 in the preferred embodiment.The extracted region is then processed to derive a corresponding imageidentifier, using the method according to GB 0719833.6. A brief summaryof this method is provided below.

A Trace transform T(d, θ) of the image region is performed by projectingall possible lines, parameterised by d, θ over the image and applying afirst functional T over these lines. The result of the Trace transformmay be decomposed to reduce its resolution in either or both of itsdimensions d, θ in accordance with the method disclosed in GB 0700468.2.A second functional P may be applied to the columns of the Tracetransform to give a vector of real numbers (i.e. a one dimensionalfunction), as in the methods of EP 06255239.3 and GB 0700468.2. Thissecond functional P is known as the diametrical functional and theresulting vector is known as the circus function.

A third functional, the circus functional, may be applied to the circusfunction to give a single number, although this step is not used in thepreferred embodiment.

Alternatively, in accordance with the method of GB 0719833.6, aso-called “band-circus” function may be obtained, as an alternative tothe circus function discussed above, which restricts the Trace transformto a subset of lines of the image, by selecting and processing valuesfrom only a part of the Trace transform.

The properties of the result can be controlled by appropriate choices ofthe first, second and/or third functional (Trace, diametrical andcircus).

A binary identifier for the image region is extracted from the circusfunction (or the band-circus function) via a frequency representation.For example, a function c(ω) may be defined on the magnitudecoefficients of the Fourier transform. One illustration of this functionis taking the difference of neighbouring coefficientsc(ω)=|F(ω)|−|F(ω+1)|  (8)

A binary string can be extracted by applying a threshold to theresulting vector, such that

$\begin{matrix}{b_{\omega} = \left\{ {\begin{matrix}{0,} & {{c(\omega)} < S} \\{1,} & {{c(\omega)} \geq S}\end{matrix}\mspace{14mu}{for}\mspace{14mu}{all}\mspace{14mu}{\omega.}} \right.} & (9)\end{matrix}$

Suitable choices for S include S=0 and S=mean(c). The binaryrepresentation of the image region is then made up of these binarystring values B={b₀, . . . , b_(n)}.

In an embodiment of the present invention, since local image regions areused (as opposed to entire images as in GB 0719833.6) modifications needto be made to the signature extraction technique described therein. Inparticular, two identifiers are extracted from the circus functions ofthe full Trace transform using the functionals:max(ξ(t)),  (10)and∫|ξ(t)′|dt.  (11)in accordance with the method disclosed in EP 06255239.3.

A further six, “multi resolution” identifiers may be extracted fromcircus functions obtained by decomposing (or sub-sampling) the distance(d) parameter of the Trace transform by a factor of 8, 16 and 32 andapplying both equations (10) and (11), in accordance with the methoddisclosed in GB 0700468.2.

A further three functions are selected from bands in the Tracetransform, in accordance with the method disclosed in GB 0712388.8, andtwo functions are extracted from the trace-annulus functions and onefunction is extracted from the trace-circular function, in accordancewith the method disclosed in GB 0719833.6.

Each of these 14 component identifiers is 8 bits, giving a total of 112bits for each feature identifier.

Additionally, a scale adapted feature direction is also calculated thatintegrates over a neighbourhood for each feature

$\begin{matrix}{{\theta\left( {x,y} \right)} = {{\tan^{- 1}\left( \frac{R_{x}\left( {x,y,\sigma_{D},\sigma_{I}} \right)}{{Ry}\left( {x,y,\sigma_{D},\sigma_{I}} \right)} \right)}.}} & (12)\end{matrix}$

The direction θ is quantised to preferably 16 levels, which can berepresented by 4 bits of data.

The image identifier or descriptor is formed from all or some of thefeature signatures. An image identifier for the complete set of featurepoints may be represented in a number of ways. In a preferred embodimentthe image descriptor is a bit stream containing a number of fields ofinformation, the first field is the number of features. Then eachfeature is represented in the bitstream as the normalised feature pointcoordinates (x and y), the quantised feature direction, followed by thebinary feature signature. Optionally, rather than store the number offeatures, padding could be used with a flag set to represent an unusedfield. The binary feature signature includes all 112 bits or optionallya subset of the 112 bits may be selected, derived or stored. Thestructure of feature identifiers is shown in FIG. 7( a) and the imagesignature in FIG. 7( b).

Image Matching using Feature Identifiers

Image matching may be performed by comparing or matching featuresignatures, which form part of the image identifiers. As mentionedpreviously, for efficiency the full matching procedure between a pair ofimages, by comparing feature signatures, is made up of four stages.Hypotheses are formed in stages one and three. A series of geometrictests are performed in stages two and four, these tests must be passedin order for a hypothesis to progress to the next stage. The stagesbecome increasingly computationally complex so each stage aims tominimise the number of hypotheses that are accepted for subsequentprocessing.

A first step involves forming hypotheses for potential matching featuresby comparing the first M out of N feature signatures from one image withthe first M out of N feature signatures from the second image. Theadvantage of using a subset of M is that it speeds up whilst maintainingrobustness. In a preferred embodiment N is 32 and M is 24.

To perform matching between two signatures B₁ and B₂, the Hammingdistance is taken:H(B ₁ ,B ₂)=ΣB ₁

B ₂,  (13)where

is the exclusive OR (XOR) operator.

In a preferred embodiment, a set of hypotheses (candidate pairs ofpotentially matching features) is generated with all combinations ofthree pairs of matching features. A hypothesis is made up a set of threepairs of feature points (a₁, a₂, a₃, b₁, b₂, b₃). A set of three pairsof features is declared a match if the distance of each feature pair isbelow a constant threshold T_(A) and the cumulative distance is belowsome second constant threshold T_(B). In a preferred hypothesisT_(A)=0.06 and T_(B)=0.2. The set of hypotheses is ordered by theircumulative distance, with the lowest distance (i.e. highest probabilityof being a match) first. In order to minimise complexity the number ofhypotheses may be limited to the K hypotheses corresponding to thelowest distance. In a preferred embodiment K is 18.

A second step applies geometric constraints to each generated from stageone. Two geometric constraints are applied to reduce false matches, i)feature direction and ii) the ratio of line lengths.

As described above, a direction is associated with each feature, thedirection for feature a_(n) is denoted θ_(an). The difference betweenangles corresponding to features from the same image is taken to ensurethat the angle distance measure is unaffected by a rotation. Thedistance measure must address the issue of left-right and top-bottomflip modification as is done here using these two measures of angledistanceΩ₁=|(θ_(a1)−θ_(a2))−(θ_(b1)−θ_(b2))| andΛ₁=|(θ_(a1)−θ_(a2))+(θ_(b1)−θ_(b2))|  (14)

Thresholds are applied to the two angle distance measures Ω₁, Λ₁ to testwhether they are in allowable intervals. Allowable values for the angledistances are dependant on the number of levels in the anglequantisation Q and are given byΩ₁ ≦q OR Q−q≦Ω ₁ ≦Q+q OR Λ₁ ≦q OR Q−q≦Λ ₁ ≦Q+q.  (15)where q is a constant that can be used to vary the sensitivity tochanges of angle. In a preferred embodiment q is set to 1 and Q is 16.

Using the locations of features selected in each hypothesis a ratio ofline lengths between the features is used as a constraint, as shown inFIG. 5. By |a₁a₂| we denote the length of the line connecting thelocation of feature a₁ to the location of feature a₂. Three featurepoints |a₁a₂|, |a₂a₃| and |a₃a₁| are used to find the ratio of lineslengths. Three line length ratios are computed

$\begin{matrix}{{L_{1} = \frac{{a_{1}a_{2}}}{{a_{1}a_{3}}}},\mspace{14mu}{L_{2} = {{\frac{{a_{2}a_{3}}}{{a_{3}a_{1}}}\mspace{14mu}{and}\mspace{14mu} L_{3}} = {\frac{{a_{3}a_{1}}}{{a_{1}a_{2}}}.}}}} & (16)\end{matrix}$

Measures of similarity (G₁, G₂, G₃) between the line length ratios fortwo images A and B are defined as:

$\begin{matrix}{{G_{1} = \frac{{L_{1}^{A} - L_{1}^{B}}}{L_{1}^{A} + L_{1}^{B}}},\mspace{14mu}{G_{2} = {{\frac{{L_{2}^{A} - L_{2}^{B}}}{L_{2}^{A} + L_{2}^{B}}\mspace{14mu}{and}\mspace{14mu} G_{3}} = {\frac{{L_{3}^{A} - L_{3}^{B}}}{L_{3}^{A} + L_{3}^{B}}.}}}} & (17)\end{matrix}$

Advantageously, the ratio of line lengths between defined points in animage are invariant to transformations such as rotation, translation andscaling, such that the above geometric constraint is suitable formatching an image that has undergone such geometric transformations.However, other suitable geometric constraints may be used, consistentwith design requirements.

In an embodiment two thresholds are applied to the distance measures,one to constrain the magnitude of the individual measures and the otherto constrain the sum of the distances.

Any hypothesis that fails one or more of the tests described above isremoved from the set of valid hypotheses. If after testing allhypotheses the set is empty the images are declared non-matching. If thehypothesis has passed all tests then it progresses to the next stage togenerate four feature hypotheses.

The third stage of matching compares all N features from the first imagewith all N features from the second image. Now, there is already ahypothesis for three feature pairs so the aim is to find a set ofhypotheses with four pairs, hypotheses that add one more feature to thecurrent three feature pair hypothesis. Potential features are foundbased on the signature distance as in stage one. The set of hypothesesare ordered by the distance of the fourth feature.

In stage four geometric analysis is carried out on the sets of fourfeature hypotheses to further reduce false acceptance rates. The firstgeometric constraint is based on the feature direction equations (14)and (15) as for three features. The ratio of lines used previously isinvariant to similarity transforms. With four features the invariancecan be relaxed to the more general affine transformation by using theratio of areas as described below. This allows tighter thresholds to beset which accept true matches whilst rejecting false matches.

From a set of four features (a₁, a₂, a₃, a₄), four triangles may beconstructed (a₁a₂a₃, a₂a₃a₄, a₃a₄a₁, a₄a₁a₂), the areas of thesetriangles can be found fromA ₁=√{square root over ((s(s−|a ₁ a ₂|)(s−|a ₂ a ₃|)(s−|a ₃ a₁|)))}{square root over ((s(s−|a ₁ a ₂|)(s−|a ₂ a ₃|)(s−|a ₃ a₁|)))}{square root over ((s(s−|a ₁ a ₂|)(s−|a ₂ a ₃|)(s−|a ₃ a₁|)))},  (18)where s=(|a₁a₂|+|a₂a₃|+|a₃a₁|)/2. Area ratios can then be calculated as

$\begin{matrix}{{A_{1/2} = \frac{A_{1}}{A_{2}}},} & (19)\end{matrix}$and the distances between an area ratio of image A and an area ratio forimage B can then be found by

$\begin{matrix}{{D\left( {A_{1/2},B_{1/2}} \right)} = {\frac{{A_{1/2} - B_{1/2}}}{A_{1/2} + B_{1/2}}.}} & (20)\end{matrix}$

Two thresholds T_(c) and T_(D) are applied to the set of four area ratiodistances between the feature sets. The first threshold rejectshypothesis which have any of the area ratios distance above a firstthreshold T_(c). The second rejects hypothesis for which the sum ofdistances is larger than a threshold T_(D). In a preferred embodimentT_(c)=0.15 and T_(D)=0.4

The image matching technique of the above described embodiments of thepresent invention may be combined with other image matching techniques,such as the method disclosed in GB 0719833.6. A descriptor is generatedfor each image that contains all of the individual identifiers. In apreferred embodiment the identifier bitstream contains two identifiersbased on GB 0719833.6, and a descriptor according to the presentinvention. Of the two identifiers based on GB 0719833.6 the first isgenerated by pre-processing the image to maintain the image aspect ratioand the other where it is processed to a square.

Preferably the matching is performed first with the faster algorithm ofGB 0719833.6 at a very low false alarm threshold. Any image pairs belowthe threshold are considered to be a match, and pairs above thethreshold (that is not considered a match by GB 0719833.6) are thenprocessed in accordance with the present invention.

An apparatus for implementing the present invention, according to anembodiment, for carrying the above methods is shown in FIG. 6. Theimplementation involves building a database of identifiers 240 forimages stored in an image database 230. The two databases 230 and 240may be the same or separated databases. The apparatus allows searchingfor an identifier 260 extracted from a query image 250 to find matchesin the identifier database 140. A, possibly ordered, list of images isreturned to the user 290 or a query application.

As the skilled person will appreciate, many variations and modificationmay be made to the described embodiments. For example, whilst thedescribed embodiments identify image regions, from which the imagesignature is derived, based on feature detection, other techniques maybe used to identify image regions that are representative of, orsignificant within, the image. Moreover, whilst the described embodimentuses a feature detector based on an image gradient method, many othersuitable methods may be used for the detection of interest points orregions in an image. Similarly, other techniques for selecting the bestor strongest detected feature or interest points are possible.

In addition, it is possible to use one or more of a variety of geometricconstraints other than the distance measures relating to the ratio oflines lengths associated with a set of three feature points, used in thedescribed embodiment. For example, any number of features points can beused in conjunction with distance measures. Alternatively, othergeometric constraints may be based on angles between feature points.Moreover, an affine model may be used to define the geometricconstraint.

It is intended to include all such variations, modifications andequivalents which fall within the spirit and scope of the presentinvention.

Alternative Implementations

Whilst the description above uses brute force searching to generatehypotheses in some situations it may be beneficial to use a structurethat allows approximate Hamming distance calculations. This wouldprovide increased search speed at the cost of some level of detection.

The feature matching approach presented is amenable to casting into aprobabilistic model along the lines presented in [reference 4 infra].

REFERENCES

-   1. Open CV Reference Manual, Chapter 1: Image Processing Gradients,    Edge and Corners—“FindCornerSubPix”:    http://opencvlibrary.sourceforge.net/CvReference-   2. D. G. Lowe, “Distinctive Image Features from Scale Invariant    Keypoints”, International Journal of Computer Vision, 60, 2, 2004.    pp. 91-110.-   3. D. G. Lowe, “Method and apparatus for identifying scale invariant    features in an image and use of same for locating an object in an    image”, U.S. Pat. No. 6,711,293, March 2000.-   4. C. Schmid, “A Structured Probabilistic Model for Recognition”,    Proc. Computer Vision and Pattern Recognition, 2, 485-490, June    1999.

The invention claimed is:
 1. A method for deriving a representation ofan image by processing signals corresponding to the image, the methodcomprising: deriving a scale-space representation of the image;processing the scale-space representation to detect a plurality offeature points having amplitude extrema; for one or more of the detectedplurality of feature points, determining a scale-dependent image regionassociated with the corresponding feature point, and deriving arepresentation for each determined, scale-dependent image region, themethod comprising selecting a predetermined number of feature pointsfrom the detected plurality of feature points, and determining ascale-dependent image region associated with each of the selectedfeature points, wherein the spatial positions of the selected featurepoints are distributed across the image, and the spatial position ofeach selected feature point is separated from the other selected featurepoints by at least a predetermined distance in the image, by selectingfeature points by order of strength with the restriction that a featurepoint is not selected if it is within an exclusion zone around any ofthe previously selected features.
 2. A method as claimed in claim 1,wherein the plurality of feature points are detected by applying anoperator based on second order image gradients, to the scale-spacerepresentation of the image, and identifying those feature points in thescale-space representation with the highest resulting values.
 3. Amethod as claimed in claim 1, where the step of selecting comprisesremoving from the detected plurality of feature points those featurepoints having the lowest amplitude values in the scale-spacerepresentation, or amplitude values in the scale-space representationbelow a threshold.
 4. A method as claimed in claim 1, wherein the stepof selecting comprises removing, from the detected plurality of featurepoints, feature points according to their spatial position in the image.5. A method as claimed in claim 1, wherein the step of selectingcomprises applying an operator to the detected plurality of featurepoints, the operator based on first order image gradients, and removingthe features points with resulting values below a threshold.
 6. A methodas claimed in claim 5, wherein the operator is a scale adapted Harrisoperator.
 7. A method as claimed in claim 1, wherein the step ofdetermining a scale-dependent image region for a feature point in thescale-space representation comprises determining a spatial region of theimage associated with the spatial position of the feature point, whereinthe size of the spatial region is based on the scale of the featurepoint.
 8. A method as claimed in claim 1, further comprising: beforedetermining a scale-dependent image region, refining the spatialpositions of the one or more feature points to sub-pixel resolution,preferably using cubic interpolation.
 9. A method as claimed in claim 1,wherein the step of deriving a representation for each determined,scale-dependent image region, comprises deriving a representation basedon line projections across the image region, preferably using the Tracetransform, or equivalent.
 10. A method as claimed in claim 1, whereinthe step of deriving a representation for each determined,scale-dependent image region, comprises: deriving a function of at leastpart of the image region; and using the function to obtain anintermediate representation of the at least part of the image region.11. A method as claimed in claim 10, comprising: for each of the one ormore determined scale-dependent image regions: deriving a function of atleast part of the image region, where the function of a translated,scaled or rotated version of an image region is a translated or scaledversion of the function of the image region.
 12. A method as claimed inclaim 10, wherein: the step of deriving a function of at least part ofthe image region comprises applying a first functional along lines overthe image region, the method preferably further comprising; applying asecond functional to the derived function to obtain an intermediaterepresentation of the at least part of the image region.
 13. A method asclaimed in claim 1, further comprising: deriving the representation ofthe image from two or more of the derived representations of thescale-dependent image regions.
 14. A method as claimed in claim 1,further comprising: for the one or more of the plurality of featurepoints, determining a feature direction for the feature point,preferably by integrating over a neighbourhood of the feature point. 15.A method as claimed in claim 14, wherein the feature direction isdetermined using the equation: $\begin{matrix}{{\theta\left( {x,y} \right)} = {{\tan^{- 1}\left( \frac{R_{x}\left( {x,y,\sigma_{D},\sigma_{I}} \right)}{{Ry}\left( {x,y,\sigma_{D},\sigma_{I}} \right)} \right)}.}} & (12)\end{matrix}$ where: θ is the feature direction of the feature point atspatial position (x, y).
 16. A method as claimed in claim 1, wherein thestep of deriving a representation for each determined, scale-dependentimage region comprises deriving a binary image descriptor for thescale-dependent image region.
 17. A method for comparing images usingdescriptors of images derived using a method as claimed in claim 1, themethod comprising: comparing representations of one or morescale-dependent image regions associated with the descriptor of a firstimage, with representations of one or more scale-dependent image regionsassociated with the descriptor of a second image.
 18. A method asclaimed in claim 17, comprising: comparing a subset of therepresentations of image regions of the descriptor of a first image witha corresponding subset of the representations of image regions of thedescriptor of a second image, to identify candidate pairs of matchingimage regions.
 19. A method as claimed in claim 17, comprising:determining a first plurality of sets of a predetermined number offeatures points associated with image regions represented in thedescriptor of the first image; determining a second plurality of sets ofthe predetermined number of features points associated with imageregions represented in the descriptor of the second image; comparing aset of the determined first plurality of sets with a set of thedetermined second plurality of sets using a matching algorithm.
 20. Amethod as claimed in claim 19, wherein the predetermined number isthree, and the matching algorithm is based on the Hamming distancebetween a pair of representations of image regions associated withfeatures points in the first and second images, and the cumulativeHamming distance between the three pairs of representations of imageregions associated with features points in the first and second images.21. A method as claimed in claim 20, further comprising: determiningcandidate matching sets of features points, in the first and secondimages, when: said Hamming distance is below a first threshold, and saidcumulative Hamming distance is below a second threshold.
 22. A method asclaimed in claim 21, further comprising: applying a geometric constraintto geometric information for each said candidate matching set, toeliminate false matches from the candidate sets.
 23. A method as claimedin claim 22, wherein the step of applying a geometric constraintcomprises: determining the length of spatial lines between each of thefeature points in the candidate sets; determining a ratio between pairsof the lines in the candidate sets of the first image and the secondimage, and deriving at least one measure of similarity between thedetermined line length ratios associated with the first image and thedetermined line length ratios associated with the second image.
 24. Amethod as claimed in claim 22, wherein the representations of the imageregions associated with the feature points include feature directioninformation, and the geometric constraint takes into account featuredirection.
 25. A method as claimed in claim 22, further comprising:eliminating a candidate matching set of feature point from the candidatesets if a measure of similarity is below a threshold.
 26. A method asclaimed in claim 22, further comprising: if there are no candidatematching sets, determining that the first and second images do notmatch.
 27. A method as claimed in claim 26, further comprising: ifcandidate matching sets remain: comparing the representations of all ofthe image regions of the descriptor of a first image with therepresentations of all of the image regions of the descriptor of asecond image, to determine a further matching set of feature points, notpresent in the candidate set.
 28. A method as claimed in claim 27,wherein a further matching set of feature points is determined based ona lowest Hamming distance between feature points of the first and secondimages, not present in the candidate sets.
 29. A method as claimed inclaim 27, further comprising: adding the determined further matching setof feature points to the candidate sets, to produce updated candidatesets, and applying a geometric constraint to geometric information foreach updated candidate matching set of four feature points, to furthereliminate false matches from the candidate sets.
 30. An apparatuscomprising: an image database for storing images or representations ofimages: an identifier database; and a processor configured to executefor executing the method of claim 1 or claim
 17. 31. A non-transitorycomputer storage medium having stored thereon instructions that, whenexecuted by a computer, perform the method as claimed in claim 1 orclaim 17.